1. Field of the Inventions
The present inventions relate to methods and apparatus for producing ions, and have particular application to structures and methods including microelectronic micro-structures used for producing ions from liquids, for example to produce ions for mass spectrometers and the like.
2. Related Art
Mass spectrometers and other analyzers have been used to determine the properties or characteristics and quantities of unknown materials, many of which are present in only minute quantities. Many such analyzers function by determining the quantity of material present in an unknown solution as a function of the relationship between the mass and the charge on ions provided to the analyzer by a source of ions. The ability of the analyzer to produce reliable results depends in part on the ability of the source of ions to produce a maximum number of individual ions for a given amount of input material.
Electro-spray ion sources are one type of source of ions for analyzers. Typical ion generation from electro-spray ion sources peaks at a certain ion generation level for a given system due to coalescing or nucleation of charged and un-charged droplets as the droplet density increases in the high electrostatic field. Most of the coalesced, larger-than-original droplets fail to eject ions from their surfaces due to new conditions and subsequently larger droplets. Larger droplets mean that their kinetic inability to reach a critical minimal volume reduces the likelihood that ions will be ejected, regardless of the liquid flow rate available for electro-spray. For example, typical liquid ion source devices have a single liquid conduit producing droplets in a range of sizes from sub-micron diameters to hundreds of microns in diameter. Ions are ejected from smaller aerosol droplets when and if the droplet reaches a critical smaller dimension and if the repulsive internal charge becomes greater than the surface tension holding the droplet in its spherical shape. Absent a critical dimension and a suitable repulsive internal charge, few or no ions are ejected. A high percentage of the droplets do not reach critical volume, resulting in a low ion yield.